This invention relates to a fuel cell which is used in an electric vehicle and the like and a manufacturing method therefor. While this invention will be described in relation to a solid-state high polymer type fuel cell, it is also applicable to a phosphoric acid fuel cell.
As is well known, the fuel cell is an apparatus having a pair of electrodes and an electrolyte sandwiched therebetween, a fuel is being supplied to one of the electrodes and an oxidant being supplied to the other of the electrodes thereby achieving an electrochemical reaction between the fuel and the oxidant within the cell to convert chemical energy directly into electrical energy.
According to the kind of the electrolyte, fuel cells are classified as including a so-called solid polymer fuel cell in which a solid polymer electrolyte membrane is used as an electrolyte, and a phosphoric acid fuel cell in which phosphoric acid is used. Recently, a solid polymer fuel cell received attention as a fuel cell which can output a nigh power. When hydrogen gas is supplied to a fuel electrode and oxygen gas is supplied to an oxidant electrode, for example, to produce an electric current in an external circuit, the following reactions as shown in the following chemical reaction formulae (1) and (2) occurs:
anode reaction: H2xe2x86x922h++2exe2x80x83xe2x80x83(1)
cathode reaction: 2H++2exe2x88x92+(1/2)O2xe2x86x92H2Oxe2x80x83xe2x80x83(2)
When these reactions occur oxygen becomes protons on the fuel cell electrode and move together with water through the electrolyte up to the oxidant electrode, where they react with oxygen on the oxidant electrode to generate water. Therefore, in order to operate the above fuel cell, it is necessary to supply and discharge the reaction gas and to take out the electric current
Some examples of the separator panel for taking out the electric current from the fuel cell and for efficiently circulating the gas and water can be found in Japanese Patent Laid-Open Nos. 58-161270, 58-161269 and 3-206763.
FIG. 15 is a sectional View for explaining the overall structure of a unit cell constituting the fuel cell disclosed in Japanese Patent Laid-Open No. 3-206763, in which reference numeral 1 and 2 designate electrically conductive separator panels, 3 is an oxidant electrode, 4 is a fuel electrode and 5 is an electrolyte member using a proton-conductive solid polymer, the electrolyte member 5, the oxidant electrode 3 and the fuel cell 4 constitute a unit cell 6.
FIG. 16 is an explanatory view showing the top surface of the separator panel of the fuel cell shown in FIG. 15, the explanation thereof will be made in conjunction with FIG. 15.
That is, the reference numeral 20 is a major surface of the separator panel 1, 21 is an electrode support portion of the separator panel 1 for supporting the electrode 3, 22 is an oxidant supply port provided in the separator panel 1 for supplying air as the oxidant, 23 is an oxidant discharge port for discharging air, 24 is a fuel supply port for supplying the fuel therethrough and 25 is a fuel discharge port for discharging the fuel.
In the separator panels 1 and 2, an oxidant flow path 10 and a fuel flow path 11 are defined by grooves cut into the main surface 20 and a space surrounded by the electrodes 3 and 4.
The operation of the fuel cell will now be described in conjunction with FIGS. 15 and 16. Oxygen supplied from the oxidant supply port 22 of the separator panel 1 is supplied to the oxidant electrode 3 through a plurality of parallel oxidant flow path 10 and the fuel is supplied to the fuel electrode 4 through the fuel gas flow path 11 in a similar manner to the flow of the oxidant. At this time, since the oxidant electrode 3 and the fuel electrode 4 are electrically connected at the outside, the foregoing chemical reaction formula (2) occurs on the side of the oxidant electrode 3 and the reduced reaction gas and water are carried through the oxidant gas flow path 11 and are discharged from the oxidant discharge port 23. On the side of the fuel electrode 4, the reaction of the above reaction formula (1) occurs and the reduced reaction gas is similarly discharged from the fuel discharge port 25 through the fuel gas flow path 11. The electrons obtained by this reaction flow from the electrodes 3 and 4 through the electrode support portions 21 and through the separator panels 1 and 2.
The oxidant flow paths 10 are formed on one of the surfaces in a serpentine manner in cross section so as to define a plurality of parallel grooves. Also, the fuel gas flow paths 11 are also a plurality of grooves similar to the oxidant flow paths 10.
In such a fuel cell, gas diffusion necessary for the reaction is promoted and the water generated at the oxidant electrode is efficiently discharged by making the boundary layer thin by increasing the gas flow speed and by making the gas flow path long by forming it into a serpentine shape.
It has also been proposed that the region be completely divided to define serpentine flow paths as shown in FIG. 17, which is a perspective view of the separator panel disclosed in Japanese patent Laid-Open No. 62-40169. In the figure, the reference numeral 7 is a separator panel, 8 and 8a are grooves, 9 and 9a are ribs. According to this measure, an inlet and an outlet for a single fluid occupy substantially the complete length of one side of the separator panel and it is difficult to provide for another fluid.
Also, a flow path in which a parallel flow path is simply folded and returned is shown in FIG. 18 which is a perspective view of a separator panel disclosed in WO96/20510. In the figure, the reference numeral 71 is an airflow path, 72 is a fuel supply port, 73 is an air supply port, 74 is an air discharge port and 75 is a fuel discharge port
In the conventional separator panels, the reaction is promoted at the region where the concentration of the fluid flowing through the flow path is high and the current density is increased within that region, making the current density over the entire separator panel non-uniform. However, this non-uniform current density is not considered at all, so that the effective reaction area is decreased, resulting in lowered properties.
In the above conventional fuel cell, the voltage generated by one unit cell is not greater than 1V and it is necessary to stack more than 100 unit cells and separator panels as disclosed in Japanese Patent Laid-Open No. 4-121914 to obtain a voltage of equal to or more than 100V which is practically necessary.
However, stacking more than 100 unit cells and separator panels at one time is not good in work efficiency and not only the precision maintenance of the alignment of the stacked elements is difficult, but also they are displaced due to vibration or the like during the operation and, in worst case gas leakage may occur due to the displacement of the gas supply port or gas discharge port.
While a fuel cell having a conventional separator panel is arranged to increase the gas flow speed to discharge the formed water as discussed above, the gas leaks between a group of the flow paths and a second group of bent flow paths next to the first group of the flow paths.
Accordingly, an object of the present invention is to provide a fuel cell which has a uniform reaction distribution over the separator panel and an improved performance.
Another object of the present invention is to provide a method for manufacturing a fuel cell in which a precisely assembled fuel cell stack can be manufactured at low cost and high efficiency.
Another object of the present invention is to provide a fuel cell that has a stable mechanical configuration during operation.
A further of object of the present invention is to provide a fuel cell in which the gas leakage can be prevented and which can be mass-produced and is capable of generating a high voltage output.
With the above object in view the fuel cell of the present invention comprises a unit cell and a separator panel alternatingly stacked on one another. The unit cell includes an electrolyte membrane sandwiched between a fuel electrode and an oxidant electrode. The separator panel includes a plurality of parallel fuel flow paths extending from a fluid supply port to fluid discharge port for supplying fluid fuel to the fuel electrode and a plurality of parallel oxidant flow paths extending from a fluid supply port to a fluid discharge port for supplying oxidant fluid to the oxidant electrode. At least the plurality of oxidant flow paths comprise a plurality of groups of parallel flow paths, which extend back and forth within divided regions of the main surface of the separator panel.
The plurality of oxidant flow paths may comprise a plurality of groups of parallel flow paths, positions along the plurality of groups of the parallel flow paths at equal distance from the respective fluid supply port thereof being distributed substantially evenly over the main surface of the separator panel.
Also, the separator panel may comprise a plurality of parallel coolant flow path for allowing a coolant to flow therethrough, and the plurality of coolant flow paths may include a plurality of groups of parallel flow paths, which extend back and forth within a region defined by a projection of the divided regions though which the oxidant flow paths extend.
The method for manufacturing a fuel cell of the present invention comprises the steps of preparing a unit cell having an electrolyte membrane sandwiched between a fuel electrode and an oxidant electrode and having a first through hole on an electrode surface, and a separator panel having a plurality of fuel flow paths extending for supplying fluid fuel to the fuel electrode and a plurality of oxidant flow paths for supplying oxidant fluid to the oxidant electrode and having a second through hole on a main surface thereof. Then, the unit cell and the separator panel are stacked one on another to make stacks and an intermediate adapter having a third through hole is inserted into the first and second through holes to hold the stacks into unit blocks. The plurality of unit blocks are stacked and a clamp shaft is inserted into the third through hole of the intermediate adapter of the stacked unit blocks to obtain a stack of the unit blocks, and then the stack of the unit blocks is clamped by the clamp shaft.
The intermediate adapter may be a cylinder having an outer diameter for allowing it to be inserted into the first and second through holes, and the third through hole may have dimensions for allowing the shaft to extend therethrough.
The first and second through holes and the intermediate adapter may have an oval cross-sectional shape.
Also, the fuel cell may comprise a unit cell having an electrolyte membrane sandwiched between the fuel electrode and the oxidant electrode, a separator panel having-formed therein fuel flow paths for supplying fuel fluid to the fuel electrode and oxidant flow paths for supplying oxidant fluid to the oxidant electrode, an intermediate adapter having a through hole therein and inserted into the unit cell and the separator panel to hold them in a stacked relationship to form unit blocks, and a clamp shaft extending through plurality of the unit blocks and clamping them into a stack of unit blocks.
The ridge width of ridges defined between grooves within the parallel flow path group may be smaller than the ridge width of the ridges defined between the grooves of the parallel flow path group adjacent to each other.
The ridge width between the adjacent groups may be increased as the distance from the folded portion increases.